core/iter/adapters/
take_while.rs

1use crate::fmt;
2use crate::iter::adapters::SourceIter;
3use crate::iter::{FusedIterator, InPlaceIterable, TrustedFused};
4use crate::num::NonZero;
5use crate::ops::{ControlFlow, Try};
6
7/// An iterator that only accepts elements while `predicate` returns `true`.
8///
9/// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
10/// documentation for more.
11///
12/// [`take_while`]: Iterator::take_while
13/// [`Iterator`]: trait.Iterator.html
14#[must_use = "iterators are lazy and do nothing unless consumed"]
15#[stable(feature = "rust1", since = "1.0.0")]
16#[derive(Clone)]
17#[ferrocene::prevalidated]
18pub struct TakeWhile<I, P> {
19    iter: I,
20    flag: bool,
21    predicate: P,
22}
23
24impl<I, P> TakeWhile<I, P> {
25    #[ferrocene::prevalidated]
26    pub(in crate::iter) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
27        TakeWhile { iter, flag: false, predicate }
28    }
29}
30
31#[stable(feature = "core_impl_debug", since = "1.9.0")]
32impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
33    #[ferrocene::prevalidated]
34    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
35        f.debug_struct("TakeWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
36    }
37}
38
39#[stable(feature = "rust1", since = "1.0.0")]
40impl<I: Iterator, P> Iterator for TakeWhile<I, P>
41where
42    P: FnMut(&I::Item) -> bool,
43{
44    type Item = I::Item;
45
46    #[inline]
47    #[ferrocene::prevalidated]
48    fn next(&mut self) -> Option<I::Item> {
49        if self.flag {
50            None
51        } else {
52            let x = self.iter.next()?;
53            if (self.predicate)(&x) {
54                Some(x)
55            } else {
56                self.flag = true;
57                None
58            }
59        }
60    }
61
62    #[inline]
63    #[ferrocene::prevalidated]
64    fn size_hint(&self) -> (usize, Option<usize>) {
65        if self.flag {
66            (0, Some(0))
67        } else {
68            let (_, upper) = self.iter.size_hint();
69            (0, upper) // can't know a lower bound, due to the predicate
70        }
71    }
72
73    #[inline]
74    #[ferrocene::prevalidated]
75    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
76    where
77        Self: Sized,
78        Fold: FnMut(Acc, Self::Item) -> R,
79        R: Try<Output = Acc>,
80    {
81        #[ferrocene::prevalidated]
82        fn check<'a, T, Acc, R: Try<Output = Acc>>(
83            flag: &'a mut bool,
84            p: &'a mut impl FnMut(&T) -> bool,
85            mut fold: impl FnMut(Acc, T) -> R + 'a,
86        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
87            move |acc, x| {
88                if p(&x) {
89                    ControlFlow::from_try(fold(acc, x))
90                } else {
91                    *flag = true;
92                    ControlFlow::Break(try { acc })
93                }
94            }
95        }
96
97        if self.flag {
98            try { init }
99        } else {
100            let flag = &mut self.flag;
101            let p = &mut self.predicate;
102            self.iter.try_fold(init, check(flag, p, fold)).into_try()
103        }
104    }
105
106    impl_fold_via_try_fold! { fold -> try_fold }
107}
108
109#[stable(feature = "fused", since = "1.26.0")]
110impl<I, P> FusedIterator for TakeWhile<I, P>
111where
112    I: FusedIterator,
113    P: FnMut(&I::Item) -> bool,
114{
115}
116
117#[unstable(issue = "none", feature = "trusted_fused")]
118unsafe impl<I: TrustedFused, P> TrustedFused for TakeWhile<I, P> {}
119
120#[unstable(issue = "none", feature = "inplace_iteration")]
121unsafe impl<P, I> SourceIter for TakeWhile<I, P>
122where
123    I: SourceIter,
124{
125    type Source = I::Source;
126
127    #[inline]
128    unsafe fn as_inner(&mut self) -> &mut I::Source {
129        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
130        unsafe { SourceIter::as_inner(&mut self.iter) }
131    }
132}
133
134#[unstable(issue = "none", feature = "inplace_iteration")]
135unsafe impl<I: InPlaceIterable, F> InPlaceIterable for TakeWhile<I, F> {
136    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
137    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
138}
core/iter/adapters/take_while.rs

core/iter/adapters/
take_while.rs
